Heat exchanger construction

ABSTRACT

A heat exchanger construction for maximum thermal fatigue life having C-shaped closure bars on the cool fluid side, the C-shaped closure bars being made from a material having a lower coefficient of thermal expansion than the remainder of the core, and having cooling fins therein and having hot fluid fins cut back from the inlet face of the core to reduce the heat input to the closure bars.

United States Patent Edward A. Rothman South Glastonbury, Conn. 873,872

Nov. 4, 1969 Aug. 24, 1971 United Aircraft Corporation East Harfiord, Conn.

HEAT EXCHANGER CONSTRUCTION 4 Claims, 3 Drawing Fm 11.8. C1 Int. Cl

Fieidoismrch inventor Appl. No. Filed Patented Assignee References Cited UNITED STATES PATENTS 9/1960 Ladd 3,241,607 3/1966 Rutledge 165/166 3,282,011 11/1966 Meserole et a1. 165/135 X 3,495,656 2/1970 Dickson 165/166 Primary Examiner-- Frederick L. Matteson Assistant Examiner-Theophi1 W. Streule A Henley-Laurence A Savage ABSTRACT: A heat exchanger construction for maximum thermal fatigue life having C-shaped closure bars on the cool fluid side, the C-shaped closure bars being made from a material having a lower coefficient of thermal expansion than the remainder of the core, and having cooling fins therein and having hot fluid fins cut back from the inlet face of the core to reduce the heat input to the closure bars.

PATENTEU AUEEMSH 4 'Q. arn 0/0 HEAT EXCHANGER CONSTRUCTION BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to heat exchanger construction and particularly to a construction for increasing the thermal fatigue life of heat exchangers.

2. Description of the Prior Art A common heat exchanger construction, well known in the art, is partially shown in FIG. 1. The heat exchanger comprises end sheet 2 and another (not shown); parting sheets 4 (also called plates); bleed air fins 6; coolant fins 8; and closures 10. It will be understood by those skilled in the art that any number of layers may be disposed, one upon the other, to form the desired heat exchanger size. The entire assembly is then brazed together to form a unit. A problem exists with this heat exchanger construction in that cracks in the closure bars and parting sheets occur and propagate longitudinally along them. For example, as shown in FIG. 1, a common type of cracking is shown: cracks l2 and 14 appear in the closure bar closest to the end sheet 2, and cracks 16 and 18 in parting sheets 6. These cracks are obviously undesirable because they cause cross leakage between the hot fluid and cool fluid passages or either fluid passage and ambient.

I have found that this cracking occurs because of the thermal stresses caused in the closure bars and parting sheets by the thermal gradients in the heat exchanger and by the differing rates of thermal expansion in the various elements making up the unit. As shown in FIG. 1, hot fluid passes over fins 6, while cool fluid passes over fins 8, the cool fluid removing heat from the hot fluid. As the hot fluid enters the heat exchanger, it immediately commences to heat the closure bars and the ends of the parting sheets. The closure bars and the parting sheets, therefore, attempt to expand. However, the end sheet, being thicker and exposed to the cool fluid, resists the expansion of the closure bars. The result is the typical cracking explained above. The novel construction described hereinafter overcomes these deficiencies and results in a heat exchanger which has greatly increased life and greatly reduced thermal fatigue cracking.

SUMMARY OF THE INVENTION An object of the present invention is a heat exchanger construction which eliminates thermal fatigue cracking in various elements of the heat exchanger unit.

ln accordance with the present invention a novel heat exchanger is provided by the combination of C-shaped closure bars for extending the transition between the closure bars and the parting sheets in the core, the C-shaped closure bars being made from a material having a lower thermal expansion coefficient than the remainder of the core materials and having fins therein to improve heat rejection capability, and hot fluid fins which are cut back to reduce the heat input to the closure bars.

My invention eliminates thermal fatigue cracking of the heat exchanger core by reducing the thermal gradients within the core and by matching the thermal expansion of the elements making up the heat exchanger core.

First, the closure bars are C-shaped. This allows the extension of the cool fluid fins into the closure bar itself so that the temperature of the closure bar is reduced; the heat is, instead, carried away by the cool fluid. Second, the C-shaped closure bars are extended in width, as compared to the square closure bars, thereby extending the transition between the closure bars and the normally thinner parting sheets. Third, the closure bars are made of a material having a lower coefficient of thermal expansion than the end sheets so that the hotter closure bars tend to expand at the same rate as the cooler end sheets, thereby eliminating the stresses set up between two pieces expanding at different rates. Fourth, the hot fluid fins are cut back from the inlet face of the heat exchanger core; this reduces the heat input to the closure bars because there is less efficient heat transfer at the surface of the heat exchanger core along the length where the hot fluid fin has been cut back.

In further accord with the present invention, l have also found that the effect of the stresses set up by the end sheets lower rate of expansion lessens towards the center of the heat exchanger core. Therefore, the finned C-shaped closure bars need only be disposed in the first few cool fluid passages in from the end sheet and in a graduated sequence, that is, wider near the end sheet and gradually decreasing in width towards the center of the core. Furthermore, the finned C-shaped closure members need be utilized only on the cool fluid inlet face.

The foregoing and other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of the preferred embodiment thereof, as illustrated in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially broken away perspective view of a heat exchanger core well known in the art.

FIG. 2 is a partially broken away perspective view of a heat exchanger core in accordance with the present invention.

FIG. 3 is a side elevation view of FIG. 2 with a portion broken away.

Referring to FIG. 2 and FIG. 3 (wherein like numerals refer to like components) a heat exchanger core is shown in accordance with the present invention. The core comprises a plurality of stacked plates and fins in a crossflow arrangement. As shown the cool fluid closure bars 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, and 44 are C-shaped and the first three to four bars closest the end sheets 20 and 22 are provided with fins 46, 48, 50, 52, 54, 56, 58, 60, 62, and 64 therein. The closure bars 30 and 44 need not be provided with fins because they are a sufficient distance from the end sheets 20 and 22 such that the difference in the thermal expansion rates between the end sheets and the closure bars and parting sheets does not affect them. Furthermore, the C-shaped closure bars decrease in width from the end sheets 20 and 22 towards the center of the heat exchanger core for the same reason. Cooling fins 63 are provided between the C-shaped closure bars.

The hot fluid passages are also provided with fins 65 which are cut back from the surface of the heat exchanger as shown so that heat transfer does not begin at the edge of the heat exchanger. The hot fluid passages are separated from the cool fluid passages by the parting sheets 66, and the hot fluid passages are closed by closure bars 68.

There has thus been described a heat exchanger construction in accordance with the present invention. 1 wish it to be understood that I do not desire to be limited to the exact details of the construction shown and described, for obvious modifications will occur to a person skilled in the art.

What I claim and desire to secure by Letters Patent of the United States is:

1. In combination with a plate-fin heat exchanger core construction for long thermal fatigue life of the type where superposed stacked plate and fin elements are adapted to form a crossflow arrangement with the plate elements and the fin elements disposed therein defining flow paths, the improvement comprising:

C-shaped closure bars having a pair of parallel legs contiguous with and separating said plates on the cool fluid side of the heat exchanger and said legs being progressively wider from the center of the core to the top and bottom of the core so as to progressively extend the transition between the closure bars and the plates; and

said fins on the hot fluid side of said heat exchanger being cut back from the inlet face of the heat exchanger core thereby reducing the heat input to the closure bars.

2. A heat exchanger core construction as recited in claim 1 wherein said C-shaped closure bars are constructed of a material having a lower thermal expansion coefficient than the remainder of the core materials.

3. A heat exchanger core construction as recited in claim 1 wherein cooling fins are disposed within at least the first C- shaped closure bar in from the end plates.

4. In combination with a plate-fin heat exchanger core construction for long thermal fatigue life of the type where superposed stacked plate and fin elements are adapted to form a crossflow arrangement with the plate elements defining flow paths and the fin elements disposed therein, the improvement comprising:

C-shaped closure bars separating said plates on the cool fluid side of the heat exchanger for extending the transition between the closure bars and plates, said C-shaped closure bars being constructed of a material having a lower thermal expansion coefficient than the remainder of the core materials and being disposed between the plates in a graduated sequence, being wider near the end sheets and gradually decreasing in width towards the center of the heat exchanger core;

cooling fins disposed within at least the first C-shaped closure bar in from the end plates and absent from the other C-shaped closure bar; and

said fins on the hot fluid side of said heat exchanger core being cut back from the inlet face of the core, thereby reducing the heat input to the closure bars. 

1. In combination with a plate-fin heat exchanger core construction for long tHermal fatigue life of the type where superposed stacked plate and fin elements are adapted to form a crossflow arrangement with the plate elements and the fin elements disposed therein defining flow paths, the improvement comprising: C-shaped closure bars having a pair of parallel legs contiguous with and separating said plates on the cool fluid side of the heat exchanger and said legs being progressively wider from the center of the core to the top and bottom of the core so as to progressively extend the transition between the closure bars and the plates; and said fins on the hot fluid side of said heat exchanger being cut back from the inlet face of the heat exchanger core thereby reducing the heat input to the closure bars.
 2. A heat exchanger core construction as recited in claim 1 wherein said C-shaped closure bars are constructed of a material having a lower thermal expansion coefficient than the remainder of the core materials.
 3. A heat exchanger core construction as recited in claim 1 wherein cooling fins are disposed within at least the first C-shaped closure bar in from the end plates.
 4. In combination with a plate-fin heat exchanger core construction for long thermal fatigue life of the type where superposed stacked plate and fin elements are adapted to form a crossflow arrangement with the plate elements defining flow paths and the fin elements disposed therein, the improvement comprising: C-shaped closure bars separating said plates on the cool fluid side of the heat exchanger for extending the transition between the closure bars and plates, said C-shaped closure bars being constructed of a material having a lower thermal expansion coefficient than the remainder of the core materials and being disposed between the plates in a graduated sequence, being wider near the end sheets and gradually decreasing in width towards the center of the heat exchanger core; cooling fins disposed within at least the first C-shaped closure bar in from the end plates and absent from the other C-shaped closure bar; and said fins on the hot fluid side of said heat exchanger core being cut back from the inlet face of the core, thereby reducing the heat input to the closure bars. 